Photonics is the science of molding the flow of light. Photonic band gap (PBG) materials, as disclosed in S. John, Phys. Rev. Lett. 58, 2486 (1987), and E. Yablonovitch, Phys. Rev. Lett. 58, 2059 (1987), are a new class of dielectrics which carry the concept of molding the flow of light to its ultimate level, namely by facilitating the coherent localization of light, see S. John, Phys. Rev. Left. 53, 2169 (1984), P. W. Anderson, Phil. Mag. B 52, 505 (1985), S. John, Physics Today 44, no. 5, 32 (1991), and D. Wiersma, D. Bartolini, A. Lagendijk and R. Righini, Nature 390, 671 (1997). This provides a mechanism for the control and inhibition of spontaneous emission of light from atoms and molecules forming the active region of the PBG materials, and offers a basis for low threshold micro-lasers and novel nonlinear optical phenomena. Light localization within a PBG facilitates the realization of high quality factor micro-cavity devices and the integration of such devices through a network of microscopic wave-guide channels (see J. D. Joannopoulos, P. R. Villeneuve and S. Fan, Nature 386, 143 (1998)) within a single all-optical microchip. Since light is caged within the dielectric microstructure, it cannot scatter into unwanted modes of free propagation and is forced to flow along engineered defect channels between the desired circuit elements. PBG materials, infiltrated with suitable liquid crystals, may exhibit fully tunable photonic band structures [see K. Busch and S. John, Phys. Rev. Left. 83, 967 (1999) and E. Yablonovitch, Nature 401, 539 (1999)] enabling the steering of light flow by an external voltage. These possibilities suggest that PBG materials may play a role in photonics, analogous to the role of semiconductors in conventional microelectronics. As pointed out by Sir John Maddox, “If only it were possible to make dielectric materials in which electromagnetic waves cannot propagate at certain frequencies, all kinds of almost magical things would be possible.” John Maddox, Nature 348, 481 (1990).
The single biggest obstacle to the realization of these photonic capabilities is the lack of a proven route for synthesis of high quality, very large-scale PBG materials with significant electromagnetic gaps at micron and sub-micron wavelengths. The method of micro-fabrication must also allow the controlled incorporation of line and point defects, for optical circuitry, during the synthetic process.
One very promising material for use in producing photonic devices is silicon. Producing photonic devices from silicon-based photonic crystals would be a very significant commercial advantage since methods of fabricating such materials could be readily retrofitted into existing silicon chip fabrication facilities.
Nature produces optically unique materials based on silica. Specifically, opals are semiprecious stones used in jewellery and decoration. The structure of naturally occuring opals was discovered for the first time in 1964 [J. V. Sanders, Nature 1964]. They are macroporous materials made by a periodic distribution of silica submicrometer spheres embedded in a silica medium with a slightly different refractive index. They present iridescent colors due to Bragg diffraction of light as a consequence of the three dimensional periodic modulation of the dielectric contrast in the structure. Owing to their potential technologic applications, the fabrication of artificial opals has become a significant goal in the field of optics.
It is very advantageous to use artificial opals as a template from which to produce inverse opals of pure silicon. In this way the periodicity of the self-assembling opal template is transferred to the inverse opal. A large scale periodic microstructure is thereby produced efficiently and at low cost, without recourse to time consuming and expensive photolithograghy (see S. John and K. Busch, Journal of Lightwave Technology IEEE, volume 17, number 11, pages 1931-1943, (1999)). Up to this point in time, conventional photolithography has produced only very small scale structures, with a very small number of repeating unit cells (see S. Y. Lin and J. G. Fleming, J. of Lightwave Technology IEEE, 17, no. 11, 1944 (1999) and S. Noda et al. ibid, 1948 (1999)). This method is effective for creating two-dimensional patterns, but does not readily lend itself to the production of large scale three-dimensional periodic structures.
It is particularly advantageous to provide a method which can produce inverse silicon opals with lattice constants spanning the range from which useful photonic devices could be produced and which at the same time is scalable to a very large number of repeating unit cells. With such a silicon inverse opal, a large number of photonic devices can be integrated into a single three-dimensional optical chip.